Generally, magnetic components use magnetic materials for shaping and directing magnetic fields in a manner designed to achieve a desired electrical performance. Magnetic components are readily used in a wide variety of electronic equipment such as computers, televisions, telephones, etc. In operation, magnetic fields may act as the medium for storing, transferring, and releasing electromagnetic energy. Transformers are one specific example of a magnetic component, and typically comprise two or more windings of conductors (e.g., copper wire) wound around a bobbin with a magnetic core inserted through the bobbin. The bobbin may generally be made of a molded plastic or any other suitable dielectric material. The conductors may be wound around the bobbin a predetermined number of times and in a predetermined configuration to achieve specific electrical characteristics. For example, the number of windings (e.g., a primary winding and a secondary winding) and the number of turns for the conductors in each winding may be a function of the intended application for the transformer.
To form the magnetic field in the transformer, a core assembly having high magnetic permeability may be inserted into the bobbin. Often the core assembly is made in two pieces, each having an “E” shaped cross-section that may be inserted into opposite ends of the bobbin. The transformer assembly may then be held together by various physical means such as a spring clip, tape, or an adhesive. Of course, different configurations may also be used for various applications.
Transformers generally operate on the principle that a change in current flowing through a first winding conductor, which is isolated from a second winding conductor, creates a magnetic flux in a core that causes a change in the current flow in the second winding conductor. The ratio of current in the two winding conductors may generally be related to the relative number of windings of each conductor. This may in turn create a voltage that may be the product of the number of turns multiplied by the change in magnetic flux.
Transformers are used in several applications, including power converters (or power adapters) used to power electronic devices, such as cell phones, computers, and the like. One type of power converter is a Switched Mode Power Supplies (SMPS). An SMPS may include a power supply unit and a circuit inside the unit to regulate the current. The regulating circuit may control the current so that it can stabilize it to a set voltage that is then sent to the electronic device. Due of weight, economic, and convenience factors, SMPS's are the devices of choice to power most consumer electronics that need stable current and voltage. However, they must be designed carefully to provide power with acceptable efficiency and minimal noise.
To meet these requirements, power converters may include one or more stages that include one or more magnetic components including filters, transformers, inductors, or the like. Many power converters are designed to provide multiple output voltages. A typical example is the desktop ATX computer power supply, which produces 12 V, 5 V, and 3.3 V as well as other supplies. The 12 V, 5 V, and 3.3 V supplies all require tight voltage regulation and must produce a large output current. In order to produce all of the desired output voltages from a single transformer, the turns-ratio of the transformer between the primary and secondary windings should match the input voltage relative to the output voltages plus any rectifier voltage drops in the output stages. In order to keep the transformer secondary turns to a minimum, some error is often introduced into the output voltages due to use of integer turns-ratios in low numbers.
As can be appreciated, it may be desirable to have relatively few secondary windings for various reasons. For example, since the voltage may be “stepped down” from the primary windings to the secondary windings (e.g., from 120 V down to 3.3 V), the turns-ratio may be very large, which requires a large number of turns for the primary windings relative to the secondary windings. Second, since the secondary windings may generally carry a relatively large amount of current, windings having a relatively large cross-section may be used, which increases the physical space required by the windings. By utilizing relatively few turns, the physical space required by the secondary windings and the primary windings may be reduced.
Some types of AC-to-DC power supplies may include isolation transformers that step a high-voltage bus (e.g. 250 V-400 V) down to one or more low-voltage, high-current outputs. The resulting large turns-ratio in the isolation transformer requires a primary winding that utilizes many turns of relatively small wire and larger, high-current secondary windings that typically contain only a few turns (e.g., less than 10-15 turns). Due to larger current requirements, the secondary windings usually have a relatively large cross-sectional area. The large cross-sectional area of the secondary windings often causes difficulty in winding the transformers and can also lead to significant high-frequency loss due to the skin effect and the proximity effect.
Due to the high current density required for high power transformers, litz wire may be utilized to wind secondary windings directly onto the bobbin. Litz wire may provide the flexibility required to maneuver the wire, but can be very costly for some applications. Additionally, the use of litz wire for multiple outputs (e.g., multiple secondary windings) requires significant hand labor and leads to poor overall copper utilization of the available space due to a large percentage of insulation in the litz wire. Additionally, the cost of manufacturing a transformer with litz wire may be relatively high and prone to manufacturing mistakes and errors.
FIGS. 1A-1C illustrate a prior art EE-type core assembly 20 and bobbin 10 used to wind an EE transformer 35. The outside diameter of a rim 16 of the bobbin 10 may be approximately the same as the outer diameter of the window area of the core assembly 20. In practice, the rim 16 of the bobbin 10 may have a diameter that is slightly smaller than the window area of the core assembly 20 to insure clearance for assembly of the core assembly 20 onto the bobbin 10 to form the transformer 35. The core assembly 20 may include a top half 22 having two outer legs 26, 28, and a center leg 30 sized to be insertable into a hollow portion of the bobbin 10. The core assembly 20 may also include a bottom half 24 having two outer legs 32, 34, and a center leg 36.
The standard method of wrapping a winding (e.g., the winding wire 38 shown in FIG. 38) for the transformer 35 is to attach a wire to a bobbin pin (e.g., a pin 14), wrap the wire around a winding surface 12 of the bobbin 10 as many times as necessary to achieve the desired number of turns, and then terminate the wire on another bobbin pin. This method may be repeated for all of the windings on the transformer 35. Additionally, one or more layers of insulation material 36 may be provided around one or more of the windings for electrical isolation.
If the transformer design maximizes the use of the available window area, then the copper winding's outermost diameter may be approximately the same as the outside diameter of the rim 16 of the bobbin 10. In the case of large current-carrying secondary windings, the windings may typically be composed of litz wire or copper foil. In either case, additional termination leads may need to be added to the litz wire or to the copper foil to connect the wire to a bobbin pin. Also, in either case, the windings of the transformer may use as much of the available window area as possible.